1.) Make a backup of all your sound files from the calibration menu --> admin settings --> transfer files screen.
2.) make your ding file, save as a PCM .wav file, 11.025kHz, 16bit, mono. Rename file to ALTITUDE.AFW, make sure the .AFW extension replaces the .WAV extension and does not instead prefix it.
3.) replace the ALTITUDE.AFW on your SD card with your custom ALTITUDE.AFW
4.) go back to transfer files screen and RESTORE all your sound files from the SD card